1. Field of the Invention
This invention relates to the field of digital copiers.
2. Description of the Related Art
A digital copier generates a digital image of an object, such as a page of printed text, and renders it onto a physical medium, such as a blank sheet of paper. Digital copiers typically include a scanner for generating the digital image of the object. The digital image typically is sent from the scanner to a computer, and ultimately to a printer, which renders the digital image onto the physical medium.
A conventional digital copier 10 is shown in FIG. 1. Conventional digital copier 10 includes conventional scanner 20 and conventional printer 40 each coupled to computer 30 by conventional bus 21. Conventional digital scanner 20 typically includes control processor 22, analog-to-digital (A/D) converter 23 and buffer 24. Printer 40 typically includes ROM 42, control processor 41, buffer 43 and print engine 44. Computer 30, typically includes memory 32 and processor 34.
Scanner 20 further includes a light source 25 for illuminating the image of the object, which can be present on, for example, document 12. Document 12 typically is placed on a platen of scanner 20 (not shown in FIG. 2). Lens 26 forms the image of document 12 onto detector 27. Detector 27 typically includes an array of elements, each of which converts incident light into a charge or analog signal. Detector 27 can be, for example, a photodiode array, a charge-coupled device array or a charge-coupled photodiode device array.
The analog signal generated by detector 27 which represents the image of document 12 is transmitted to A/D converter 23 for conversion into a digital image signal. The digital image signal from A/D converter 23 is coupled to control processor 22. Control processor 22 typically includes hardware and software designed to manipulate the digital image signal from A/D converter 23, for example to filter the digital image signal and to improve image quality. For example, control processor 22 typically performs image enhancement and noise removal on the digital image signal.
Embodiments of control processor 22 also provide signal compression, especially where the digital image signal representing the image of document 12 comprises a large number of bits, such as when document 12 includes color graphics or the like. Control processor 22 also can generate color transforms as necessary, such as by transforming an image in the RGB color space to the YUV or YIQ color space, and may subsample as necessary as a means for compressing the transformed image.
The relatively high latency and low bandwidth of bus 21 limits the rate of transfer of digital image signals from scanner 20 to computer 30 to such an extent that interim storage of the digital image signals is required. On-board buffer 24 is provided for the interim storage. Digital image signals from buffer 24 are transmitted to computer 30 over bus 21 at a rate allowed by bus 21 after bus 21 has been acquired by scanner 20, i.e., after expiration of the latency period of bus 21. When scanning multiple documents in relatively rapid succession, digital image signals from a plurality of images will tend to accumulate in buffer 24, because the latency and bandwidth limitations of bus 21 prevent real-time transfer of the signals. A danger exists that buffer 24 will fill to capacity, resulting in an inability to add additional signals and a loss of some digital image signals. Moreover, the need for including buffer 24 in the hardware of scanner 20 adds to the cost of manufacturing copier 10.
Digital image signals transmitted over bus 21 to computer 30 are transmitted by computer 30 over bus 21 to printer 40. Printer 40 renders digital image signals onto a physical medium.
Continuing to refer to FIG. 1, conventional printer 40 of copier 10 includes control processor 41, read-only memory (ROM) 42, buffer 43 and print engine 44. Generally, during the copying process, digital image signals are transmitted from computer 30 over bus 21 to printer 40. The digital image signals enter the printer and typically are stored in a memory buffer, such as buffer 43.
While in buffer 43, control processor 41 can sort the digital image signals to ensure their correct orientation on the physical medium when rendered. Digital image signals typically remain in buffer 43 until the buffer accumulates a complete page of digital image signals to be rendered, at which point the bit-mapped graphics and text digital image signals can be rendered by print engine 44. Control processor 41 typically arranges digital image signals in the buffer 43 into sequential horizontal lines for printing.
Control processor 41 in conjunction with software stored, for example, in ROM 42, further is capable of manipulating any digital image signals from computer 30 prior to rendering, for example to improve image quality. For example, control processor 41 can perform image enhancement on the digital image signals. Control processor 41 also might decompress a compressed digital image signal transmitted from scanner 20, for example, representing a complex graphics image or the like.
It is evident, therefore, that real-time rendering in conventional copiers depends on the timely provision of print head signals to the print engine of the printer element of the copier. Buffer 43 must be instantly accessible by print engine 44 and filled with digital image signals when print engine 44 is ready to accept data and print. In a conventional digital copier in which the printer element is coupled to the computer through a conventional bus, real time rendering often is unattainable because the relatively high latency and low bandwidth of bus 21 limits the amount of real-time signal transfer to printer 40 from computer 30. In essence, digital image signals generally are not instantly accessible by the print engine in real time. This is especially true for complex or color graphics which require representation by a significant number of bits.
As explained above, real-time rendering often is unattainable in conventional copiers, such as copier 10. Digital image signals generated at a scanning element of the copier, such as scanner 20, back-up into buffer 24 while bus 21 is being acquired. Even after the acquisition of bus 21 after the elapse of its period of latency, bus 21, because of limitations in its maximum rate of digital signal transfer (bandwidth), often is incapable of substantially immediately transmitting all of the digital image signals associated with an image of an object scanned by scanner 20. High latency and low bandwidth constraints imposed by bus 21 between computer 30 and printer 40 additionally affect an ability of copier 10 to render digital images in real time. Typically, acquisition of bus 21 by computer 30 will be delayed and digital image signals can not be transferred fast enough to suit the needs of print engine 44 to print the object in real time.